Scanning optical apparatus

ABSTRACT

By magnifying a beam light from a polarizer by a scanning lens (also referred to as a magnifying lens) and a decentered lens (serving as a correcting lens) which is a lens arranged in a side of a scanning surface with an axis thereof decentered relative to a light axis of the beam light, a spherical aberration normally exacerbated by magnifying the beam light by the scanning lens is suppressed. In addition, even if the scanning lens and the correcting lens are arranged close to the polarizer, main scanning magnification is kept small and spherical aberration is suppressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a scanning optical apparatus forscanning a light outputted from a light source on a prescribed scanningsurface.

2. Description of the Related Art

In image forming apparatuses such as printers, copiers, and facsimiles,a scanning optical apparatus, which scans a beam light for writing anelectrostatic latent image on an image supporter, is employed for thepurpose of writing said electrostatic latent image on said imagesupporter such as photoreceptor drums.

Such scanning optical apparatus employs a polarizer such as polygonmirrors for converging the beam light into scanning light. The beamlight from a light source is converged on the surface of the polarizer,and then, converged again on the image supporter (hereinafter referredto as “photoreceptor drum”) by a lens (so called, f. theta. lens). Thatis, the beam light is coupled in relation to the surface of thepolarizer and the photoreceptor drum, thereby correcting an optical facetangle error of the polarizer. Additionally hereinafter, a scanningdirection of the beam light by the polarizer is referred to as “mainscanning direction”, and a direction at right angles to the travelingdirections of the main scanning direction and the beam light is referredto as “sub scanning direction”.

In recent years, as a lens for converging the beam light on the imagesupporter, for example, a plurality of lenses (such as, a scanning lensand a correcting lens) are employed, arranged on the light path of asingle beam light as illustrated in a patent literature 1 (UnexaminedJapanese Patent Publication No. H9-80331).

As indicated in Patent literature 1, the followings are benefited byusing a plurality of lenses. That is, when a plurality of lenses isused, controllable parameters generally increase, thereby making it easyfor the optical design to be adaptable to various conditions.

For example, spherical aberration can generally be suppressed small,when the beam light fallen on a lens has a narrower spread (in short,the beam light should be fallen on as near the center of the lens aspossible). Therefore, when using a plurality of lenses, it becomespossible to employ a method for converging the beam light in a phasedmanner by such plurality of lenses, and thus, the spherical aberrationof when the beam light is converged on the photoreceptor drum issuppressed smaller than that of when the beam light is converged by asingle lens. This enables density growth of the beam light on thephotoreceptor drum, thereby increasing the writing speed of theelectrostatic latent image.

FIG. 1, as illustrated in Patent literature 1, shows a schematiccross-sectional view of a printer B (image forming apparatus) employinga scanning optical apparatus X2 according to a conventional example, inwhich a plurality of lenses is arranged in between a polarizer and aphotoreceptor drum. Hereinafter, as referring to FIG. 1, Scanningoptical apparatus X2 in accordance with a conventional example, as wellas Printer B using the same are explained.

Printer B shown in FIG. 1 comprises a printing member α1 for forming atoner image and printing it onto printing paper, a paper feeder α2 forfeeding the printing paper to Printing member α1, and a paper dischargerα3 for discharging the printing paper on which printing has beenconducted. Through an external input interface not shown, a prescribedprinting request signal indicating a printing request, as well as aimage data signal indicating image data are inputted from an externaldevice (typically, a personal computer) connected to Printer B. Theimage data is read by an image processing controller not shown based onthe image data signal, and then transformed into gray value datarelative to each of four colors: black (BK), magenta (M), yellow (Y),and cyan (C).

Printing member α1 schematically comprises, such as; photoreceptor drums1BK, 1M, 1Y, and 1C corresponding to each of said four colors; Scanningoptical apparatus X2; developers 7BK, 7M, 7Y, and 7C corresponding toeach of the colors; a intermediate transfer belt 8; various types ofrollers 9 a, 9 b, and 9 c; and a fixing apparatus 10. Said imageprocessing controller controls four light sources 2 (see FIG. 7, blacklight source 2BK, magenta light source 2M, yellow light source 2Y, andcyan light source 2C) based on the gray value data for illuminating alight onto each of Photoreceptor drums 1 (black Photoreceptor drum 1BK,magenta Photoreceptor drum 1M, yellow Photoreceptor drum 1Y, and cyanPhotoreceptor drum 1C) which correspond to four colors black (BK),magenta (M), yellow (Y), and cyan (C), thereby illuminating a beam.

The beam is guided to the above mentioned each of Photoreceptor drums 1by Scanning optical apparatus X2 having such as a plurality ofdeflecting mirrors 3, polarizer 4, and each of lenses 5, 6 as describedlater in details, thereby forming an electrostatic latent image on thesurface of each Photoreceptor drum 1.

Additionally, the toner on developing rollers in Developers 7 (blackDeveloper 7BK, magenta Developer 7M, yellow Developer 7Y, and cyanDeveloper 7C) corresponding to each of Photoreceptor drums 1 is pulledonto the surface of each of Photoreceptor drums 1, and then, by thetoner, an electrostatic latent image is developed as a toner imageaccording to the electric potential gap (developing bias) between eachof Photoreceptor drum 1 and each of the developing rollers.

Paper feeder α2 schematically comprises such as a paper cassette 11 anda paper feeding roller 12. Printing paper is previously set in Papercassette 11. According to a printing request from a user (for instance,an operation input from an operation panel installed in the exterior ofPrinter B), Paper feeding roller 12 is rotary-driven by the control ofthe image processing controller, thereby delivering the printing paperin Paper cassette 11 into Printing member α1.

The printing paper from Paper feeder α2 is delivered by a deliveringroller 9 a. Also, on a registration roller 9 b, the printing paper isset in the suspended state for a proper time. This enables adjustment oftiming of the printing paper reaching to a nip between Intermediatetransfer belt 8 and a secondary transfer roller 9 c. On the other hand,the toner image formed on each of the Photoreceptor drums 1 istransferred to Intermediate transfer belt 8, and then, by the drive ofthe same, transferred onto the printing paper passing through the nipbetween Intermediate transfer belt 8 and Secondary transfer roller 9 c.Then, the printing paper on which the toner image was transferred isdelivered to Fixing apparatus 10, and then fixed onto the printing paperby, for example, such as a heat roller. The printing paper on which thetoner image was fixed is then delivered to Paper discharger α3 anddischarged.

Scanning optical apparatus X2 is for guiding each of the beam lightsoutputted from a plurality of Light sources 2 for writing anelectrostatic latent image to each of corresponding Photoreceptor drums1, and at the same time, for scanning said beam lights thereon.

FIG. 7 shows a general structure of Scanning optical apparatus X2.Hereinafter, as referring to FIGS. 1 and 7, Scanning optical apparatusX2 in accordance with a conventional example is explained. In addition,as mentioned above, Scanning optical apparatus X2 is applicable totandem type Printer B, in which totally four light paths for guiding thebeam light to each of four Photoreceptor drums 1 (1BK, 1M, 1Y, and 1C)are formed. However, in FIG. 7, one of four light paths ishypothetically shown for simplicity.

Scanning optical apparatus X2 includes; Light sources 2 corresponding toeach of the above-mentioned four colors (Black light source 2BK, Magentalight source 2M, Yellow light source 2Y, and Cyan light source 2C);collimator lenses 13 corresponding to each of the four colors (blackcollimator lens 13BK, magenta collimator lens 13M, yellow collimatorlens 13Y, and cyan collimator lens 13C); an aperture 14; a cylindricallens 15; a polarizer 4; a scanning lens 5 common between the fourcolors; correcting lenses 6 corresponding to each of the four colors(black correcting lens 6BK, magenta correcting lens 6M, yellowcorrecting lens 6Y, and cyan correcting lens 6C). Scanning opticalapparatus X2 also includes such as one or a plurality of deflectingmirrors corresponding to each of the four colors (black deflectingmirror 3BK1, magenta deflecting mirrors 3M1, 3M2, and 3M3, yellowdeflecting mirrors 3Y1 and 3Y2, and cyan deflecting mirrors 3C1 and3C2), however not shown in FIG. 7.

The beam light outputted from each of Light sources 2 is transformedinto a parallel light (the light with no diameter changes relative tothe traveling direction) by passing through Collimator lens 13. Also,the beam light is shaped by passing through Aperture 14. Furthermore,the beam light passes through Cylindrical lens 15, and by the lightcondensing effect thereof, converges near the surface of Polarizer 4,such as polygon mirrors or MEMS (MicroElectroMechanical system) mirrors.Polarizer 4 rotates about its rotating shaft center 4 a, and therebytransforming the beam light into a scanning light.

FIG. 8 shows a cross-sectional view along a sub scanning direction of ascanning optical apparatus X1 according to an embodiment of the presentinvention, and more particularly, a cross-sectional view along abisector S2-S2 of the scanning range of the beam light shown in FIG. 7.

As shown in FIG. 8, the beam light converged and reflected on or nearPolarizer 4 falls on Scanning lens 5, and then is refracted by Scanninglens 5 such that, after the output, the light flux is reduced in the subscanning direction along with the progression. Also, as being reduced,the beam light falls on each of Correcting lenses 6 corresponding toeach of the colors. Correcting lens 6 is a spherical surface shape lenswith its cross-sectional shape in the sub scanning direction having afixed curvature. The beam light is converged on the surface of eachPhotoreceptor drum 1 by refraction of Correcting lens 6. With the beamlight which converges in this way scanned on the surface of eachPhotoreceptor drum 1, an electrostatic latent image is written on eachPhotoreceptor drum 1.

In the above-mentioned structure, it is possible to suppress sphericalaberration of Correcting lens 6 by gradually converging the beam lightby means of Scanning lens 5 and Correcting lens 6, thereby realizingdensity growth of the beam light on each Photoreceptor drum 1. Thisenables improvement of the writing speed and the image quality of anelectrostatic latent image.

However, the following problems are still concerned in theabove-mentioned conventional example.

As has been well-known, sub scanning magnification β between Polarizer 4and Photoreceptor drum 1 (the ratio between the size of the image onPolarizer 4 and the size of the image on Photoreceptor drum 1) dependson the ratio between a distance T from Polarizer 4 to the scanningposition of the beam light on Photoreceptor drum 1 and a distance L3from Polarizer 4 to the reduction starting point at which the beam lightstarts reducing. Particularly, in general, the smaller T is relative toL3, the larger the sub scanning magnification β increases. Additionally,in the case of FIG. 7, the distance L3 equals to the distance L2 fromPolarizer 4 to Scanning lens 5.

When the sub scanning magnification β increases, the following problemsoccur. That is, as shown in FIG. 8, the incident position of the beamlight on Polarizer 4 is displaced from the point A1 to the point B1 forthe amount of ΔX. Accompanying with such displacement of the incidentposition of the beam light relative to Polarizer 4, displacement for theamount of ΔS may occur also in the incident position of the beam lightrelative to Photoreceptor drum 1. Such relationship between thedisplacement amounts of ΔX and ΔS can be represented in the followingexpression (1)ΔS=|β|ΔX  (1)

In short, the sub scanning magnification β is a magnification ratio ofthe displacement of the beam light, and when such sub scanningmagnification β is large, the displacement of the beam light in the subscanning direction on Photoreceptor drum 1 becomes large. This makes itdifficult to keep the scanning path of the beam light to be linear onPhotoreceptor drum 1 (so-called field curvature becomes large), andcannot maintain the quality of an image formed in an image formingapparatus.

In order to maintain the sub scanning magnification β small, the beamlight reduction should be started as far from Polarizer 4 as possible(magnifying L3 relative to T) with Scanning lens 5 put away fromPolarizer 4, however, the following problems may occur in such case.

In FIG. 9, the cross-section in the main scanning direction of Scanningoptical apparatus X2 is illustrated in two ways: (a) when Scanning lens5 is close to Polarizer 4, (b) when Scanning lens 5 is far fromPolarizer 4.

As shown in FIG. 9( a), when Scanning lens 5 is close to Polarizer 4,Scanning lens 5 short in the main scanning direction can be used, sinceScanning lens 5 deflects the beam light before the scanning range of thebeam light scanned by Polarizer 4 is magnified in the main scanningdirection. Similarly, Correcting lens 6 short in the main scanningdirection can be used.

On the other hand, as shown in FIG. 9( b), when Scanning lens 5 ispulled away from Polarizer 4, Scanning lens 5 which is long enough forthe scanning range of the beam light widely magnified in the mainscanning direction is needed. Therefore, when Scanning lens 5 is pulledaway from Polarizer 4 in order to maintain the sub scanningmagnification β small, Scanning lens 5 is elongated in the maindirection. Additionally, since Correcting lens 6 is generally longerthan Scanning lens 5 in the main scanning direction, the elongation ofScanning lens 5 is synonymous with the elongation of Correcting lens 6,and consequently, resulting in size growth of the entire scanningoptical apparatus.

As above, in the conventional example, it was impossible to strike abalance between maintaining the sub scanning magnification β small(decreasing field curvature) and downsizing the size of a scanningoptical apparatus.

Consequently, this invention has been invented considering the foregoingconditions, and the purpose of this invention is to provide a scanningoptical apparatus capable of maintaining the sub scanning magnificationβ small, and at the same time, keeping the size of the apparatus smallwithout elongating a lens.

SUMMARY OF THE INVENTION

In order to achieve the above purpose, this invention provides ascanning optical apparatus comprising a first lens system for converginga beam light outputted from a prescribed light source which outputs abeam light, wherein said beam light is scanned on a prescribed scanningsurface by a beam light scanning means such as polygon mirrors(hereinafter referred to as ‘polarizer’) arranged in or near aconverging point of said beam light defined by said first lens system,and wherein said beam light to be scanned by a second lens systemprovided in between said polarizer and said scanning surface isconverged on said scanning surface. Said second lens system includes alens in the side of said polarizer corresponding to Scanning lens 5 inthe description of a conventional example and a lens in the side of saidscanning surface corresponding to Correcting lens 6 in the descriptionof a conventional example, wherein said beam light is magnified by alens in the side of said polarizer in a sub scanning direction whichruns at right angle to a main scanning direction as a scanning directionof said beam light on said scanning surface, and said beam light isconverged on said scanning surface as being reduced in said sub scanningdirection by a lens (hereinafter referred to as ‘decentered lens’),which is in the side of said scanning surface and arranged as beinginclined and/or decentered relative to the light axis of said beamlight.

This structure shifts a reduction starting point in the sub scanningdirection of the beam light from the side of the magnifying lens farfrom the scanning surface to the side of the decentered lens, andtherefore, it is possible to maintain the sub scanning magnification βsmall even the magnifying lens and the decentered lens forming thesecond lens system are moved relatively closer to the side of thepolarizer. By maintaining the sub scanning magnification β small, itbecomes easy to suppress field curvature on the scanning surface of thebeam light (that is, it becomes easy to keep the scanning line to be abeautiful linear), thereby writing a high-quality electrostatic latentimage. Additionally, it is still possible to arrange the second lenssystem (the magnifying lens and the decentered lens) relatively in theside of the polarizer as maintaining the sub scanning magnification βsmall. Therefore, the magnifying lens and the decentered lens can beprovided only to cover the scanning range of the beam light not yetspreading too wide, that is, these lenses short in the main scanningdirection can be employed. This enables the downsizing of the apparatus.

In addition, when the beam light is magnified in the sub scanningdirection, this makes it difficult to suppress the spherical aberrationon the scanning surface. However, by employing a decentered lensdecentered relative to the light axis of the beam light, that is,arranged with its lens axis displaced (so called, shift) or with itslens axis inclined relative to the light axis (so called, tilt), it ispossible to maintain spherical aberration small which is aggravated bymagnification by the magnifying lens, and thereby enabling densitygrowth of the beam light on the image supporting member.

Here, due to manufacturing purpose, a lens of a figure, having aspherical shape formed so as to have nearly fixed curvature of incidentsurface and the output surface of the beam light is preferred as thedecentered lens.

In order to suppress the aggravated spherical aberration, instead of amethod for decentering arrangement of a lens as described the presentinvention, a lens in an aspherical shape may rightly be used, however,the manufacturing of the same is generally difficult. On the other hand,in the method of decentering a lens as described in the presentinvention, it is often possible to sufficiently suppress the sphericalaberration even said lens having a spherical surface shape. Also, sincea general spherical lens only has to be arranged so as to be decenteredrelative to the light axis of the beam light, no difficulty occurs inmanufacturing.

As mentioned above, the point of this invention is that, sub scanningmagnification β is suppressed small by magnifying the beam light in thesub scanning direction by means of the magnifying lens, as realizingshortening the length of each lens as well as downsizing of theapparatus by arranging the second lens system near the scanning means,while realizing density growth of the beam light by suppressingspherical aberration aggravated by magnification of the beam light smallby employing a decentered lens arranged as being decentered relative tothe beam light as a lens in the downstream from the magnifying lens.

Here, the magnifying lens is preferred to have a negative refractivepower which is capable of magnifying a parallel beam light with nodiameter changes relative to its traveling direction. By using suchmagnifying lens having a negative refractive index, the sub scanningmagnification β can be reduced to a still smaller value.

On the other hand, the beam light is converged on or near the polarizer,and moves in the magnifying lens from the polarizer as magnifying itsdiameter. Consequently, without such a positive refractive power strongenough to turn the beam light magnifying its diameter into reducing thesame, the diameter of the beam light after passing through themagnifying lens is magnified, and therefore, the refractive power of themagnifying lens is not necessarily negative.

Furthermore, when L1 is a distance between the polarizer (the beam lightscanning means) and the decentered lens on the light axis of the beamlight, and T is a distance between the polarizer and the scanningsurface, and if the following expression (2) is satisfied, sufficientdownsizing of the apparatus can be achieved, and moreover, the subscanning magnification β can be maintained small.0.3≦L1/T≦0.5  (2)

Additionally, within the range in which the above conditions aresatisfied, a condition: |β|≦2 necessary for writing a high-qualityelectrostatic latent image is satisfied. Also, spherical aberration canbe suppressed small.

In addition, this invention is applicable to a multi-bean scanningoptical apparatus using a plurality of light sources which respectivelyoutputs the beam light.

According to the present invention, since a reduction starting point inthe sub scanning direction of a beam light shifts from a magnifying lensside far from a scanning surface of the beam light to the scanningsurface side, it is possible to maintain the sub scanning magnificationβ small, leaving the magnifying lens and the decentered lens arrangednear a polarizer. This allows the field curvature on the scanningsurface of the beam light to be suppressed easily, thereby writing ahigh-quality electrostatic latent image. Additionally, arranging themagnifying lens and the decentered lens near the polarizer, it ispossible to realize downsizing of the apparatus without elongating theselenses in the main scanning direction.

In addition, when the beam light is magnified in the sub scanningdirection, it becomes difficult to suppress the spherical aberration onthe scanning surface. However, by employing a decentered lens arrangedwith its lens axis displaced (so called, shift) relative to the lightaxis of the beam light, or with its leas axis inclined (so called, tilt)relative to the light axis, it is possible to maintain sphericalaberration small which is aggravated by magnification by the magnifyinglens, and thereby enabling density growth of the beam light on the imagesupporter.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading of the followingdetailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general structure of an image forming apparatus includinga scanning optical apparatus according to an embodiment of the presentinvention;

FIG. 2 shows a cross-sectional view including a scanning direction of ascanning optical apparatus according to an embodiment of the presentinvention;

FIG. 3 shows a cross-sectional view along a sub scanning direction of ascanning optical apparatus according to an embodiment of the presentinvention;

FIG. 4 shows a graph for describing converging effect of a beam lightfrom a scanning optical apparatus according to an embodiment of thepresent invention;

FIG. 5 shows a schematic diagram for describing a relationship betweenrefractive power of a scanning lens and diameter change of beam light;

FIG. 6 shows a schematic diagram for describing displacement of ahypothetical polarizer;

FIG. 7 shows a cross-sectional view along a main scanning direction of ascanning optical apparatus according to a conventional example;

FIG. 8 shows a cross-sectional view along a sub scanning direction of ascanning optical apparatus according to a conventional example;

FIG. 9 shows a schematic diagram for describing length change of a lensrelative to the distance from a polarizer;

FIG. 10 shows a schematic diagram for describing the reason why thepresent invention achieves downsizing of the apparatus;

FIG. 11 shows a schematic diagram for describing definition of tilt andshift.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With embodiments of the present invention described hereinafter withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variouschanges and modifications may be effected therein by one skilled in theart without departing from the scope or spirit of the invention asdefined in the appended claims.

A printer A shown in FIG. 1 is a tandem type printer havingphotoreceptor drums 1BK, 1M, 1Y and 1C corresponding to each of fourcolors: black (BK), magenta (M), yellow (Y), and cyan (C). Printer Aincludes a scanning optical apparatus X1 according to one embodiment ofthe present invention which writes an electrostatic latent image on eachof Photoreceptor drums 1BK, 1M, 1Y, and 1C.

The feature of Printer A is it comprises a scanning optical apparatus X1according to one embodiment of the present invention, and other partsare not described here since having no relationship with the presentinvention.

Hereinafter, as referring to FIG. 2, the feature of a scanning opticalapparatus X1 according to one embodiment of the present invention isdescribed in details.

Scanning optical apparatus X1 accommodates with tandem type Printer A,and that is, light paths are formed therein for guiding a beam light toeach of four Photoreceptor drums 1. However, only one of four lightpaths is hypothetically indicated here in FIG. 2 for simplicity.

Scanning optical apparatus X1 comprises; Light sources 2 correspondingto each of the above-mentioned four colors (black light source 2BK,magenta light source 2M, yellow light source 2Y, and cyan light source2C); collimator lenses 13 corresponding to each of the four colors(black collimator lens 13BK, magenta collimator lens 13M, yellowcollimator lens 13Y, and cyan collimator lens 13C); an aperture 14; acylindrical lens 15; a polarizer 4 (one example of a beam light scanningmeans); a scanning lens 16 common between the four colors (one exampleof a magnifying lens); correcting lenses 17 corresponding to each of thefour colors (one example of a decentered lens: black correcting lens17BK, magenta correcting lens 17M, yellow correcting lens 17Y, and cyancorrecting lens 17C).

Scanning optical apparatus X1 also includes such as one or a pluralityof deflecting mirrors corresponding to each of the four colors (blackdeflecting mirror 3BK1, magenta deflecting mirrors 3M1, 3M2, and 3M3,yellow deflecting mirrors 3Y1 and 3Y2, and cyan deflecting mirrors 3C1and 3C2), however not shown in FIG. 2.

The beam light outputted from each of Light sources 2 is transformedinto a parallel light (the light with no diameter changes relative tothe traveling direction) by passing through Collimator lens 13. Also,the beam light is shaped by passing through Aperture 14. Furthermore,the beam light passes through Cylindrical lens 15, and by the lightcondensing effect thereof, converges near the surface of Polarizer 4,such as a polygon mirror or a MEMS mirror. Polarizer 4 rotates about itsrotating shaft center 4 a, and thereby transforming the beam light intoa scanning light for scanning the surface of each Photoreceptor drum 1(one example of a scanning surface). Foregoing is similar to theconventional example. Collimator lens 13 and Cylindrical lens 15 are oneexample of a first optical system. And also, Polarizer 4 is one exampleof a beam light scanning means.

Here, in Scanning optical apparatus X1 according to one embodiment ofthe present invention, Scanning lens 5 and Correcting lens 6 in Scanningoptical apparatus X2 according to the conventional example are replacedrespectively with Scanning lens 16 and Correcting lens 17. With suchstructural difference, two requirements of downsizing of the apparatusand reducing the sub scanning magnification β (reducing field curvature)which are antinomy in the conventional example can be combined inScanning optical apparatus X1 according to one embodiment of the presentinvention.

In FIG. 10, a schematic diagram for describing the reason why thepresent invention achieves downsizing of the apparatus is illustrated.Particularly, FIG. 10 shows a cross-section of the main scanning nearPolarizer 4 in Scanning optical apparatus X2 according to theconventional example, as well as a cross-section of the main scanningnear Polarizer 4 in Scanning optical apparatus X1 according to oneembodiment of the present invention, in the same figure in the samescale. Hereinafter, as referring to FIG. 10, the reason why Scanningoptical apparatus X1 according to one embodiment of the presentinvention achieves downsizing of the apparatus is described.

As mentioned above, the sub scanning magnification β between Polarizer 4and Photoreceptor drum 1 depends on the ratio between a distance T fromPolarizer 4 to Photoreceptor drum 1 and a distance L3 from Polarizer 4to the reduction starting point where a light flux of the beam lightstarts to reduce in the sub scanning direction, and more particularly,the larger the ratio of L3 in T is, the smaller the sub scanningmagnification β becomes.

In Scanning optical apparatus X2, the reduction starting point of lightflux of the beam light is fixed to the position of Scanning lens 5arranged in the side of Polarizer 4. On the other hand, in Scanningoptical apparatus X1, from the reason described later, the reductionstarting point of light flux of the beam light is fixed not to theposition of Scanning lens 16, but to the position of Correcting lens 17arranged in the side of Photoreceptor drum 1. Here, as shown in FIG. 10,even when Correcting lens 17 is arranged in the position of Scanninglens 5 in Scanning optical apparatus X2, the sub scanning magnificationβ same as that of Scanning optical apparatus X2 can be obtained inScanning optical apparatus X1.

That is, since it is possible to arrange Scanning lens 16 and Correctinglens 17 near Polarizer 4, Scanning lens 16 and Correcting lens 17 can beprovided only to cover the scanning range of the beam light which is notyet spreading too wide. And as shown in FIG. 10, it is thereforeapparent that Scanning lens 16 and Correcting lens 17 used in Scanningoptical apparatus X1 can be shorter in the main scanning direction thanScanning lens 5 and Correcting lens 6 used in Scanning optical apparatusX2 of the conventional example. Such reduction in lens lengthcontributes to the downsizing of Scanning optical apparatus X1.

FIG. 3 shows a cross-sectional view along a sub scanning direction ofScanning optical apparatus X1 according to an embodiment of the presentinvention, and more particularly, shows a cross-sectional view along abisector S1-S1 of the scanning range of the beam light shown in FIG. 2.Hereinafter, as referring to FIG. 3, the feature point of Scanningoptical apparatus X1 according to one embodiment of the presentinvention is explained. Additionally, although the following explanationuses a cross-sectional view along the bisector S1-S1, it is needless tosay that the following explanation can be approved at least in anarbitrary position within the scanning range of the beam light in themain scanning position.

As shown in FIG. 3, a light flux of the beam light scanned in the mainscanning direction by Polarizer 4 (beam light scanning means) isconverged in the sub scanning direction on the surface (scanningsurface) of Photoreceptor drum 1 by means of Scanning lens 16 inPolarizer 4 side and Correcting lens 17 in Photoreceptor drum 1 side.Scanning lens 16 and Correcting lens 17 are one example of the secondlens system.

Here, on the surface of Polarizer 4, the light flux of the beam light isonce converged by Cylindrical lens 15 (see FIG. 2), and the beam lightfrom Polarizer 4 then falls on Scanning lens 16 as magnifying itsdiameter along with the progression.

In the conventional example, Scanning lens 5 has a strong positiverefractive power, and thus, the beam light falling on Scanning lens 5 asmagnifying its diameter becomes a light which proceeds while reduces byrefraction of Scanning lens 5 when outputted from Scanning lens 5. Onthe other hand, Scanning lens 16 in Scanning optical apparatus X1according to an embodiment of the present invention has a negativerefractive power, and as shown in FIG. 3, further magnifies the beamlight falling on as magnifying its diameter in the sub scanningdirection. Scanning lens 16 is one example of a magnifying lens.

In the conventional example, the reduction starting point of the lightflux of the beam light is fixed to the position of Scanning lens 5arranged in the side of Polarizer 4. On the other hand, as in Scanningoptical apparatus X1 according to an embodiment of the presentinvention, the reduction starting point of the light flux of the beamlight is determined according to the position of Correcting lens 17, notScanning lens 16, by magnifying the light flux of the beam light in thesub scanning direction by means of Scanning lens 16. Thus, the distanceL3 between Polarizer 4 and the reduction starting point becomes equal toa distance L1′ from Polarizer 4 to Correcting lens 17, not to thedistance L2 between Polarizer 4 and Scanning lens 16 as in theconventional example. Consequently, the ratio of the distance L3 fromPolarizer 4 to the reduction starting point in the distance T fromPolarizer 4 to Photoreceptor drum 1 becomes greater, and as a responseto this, the sub scanning magnification β becomes smaller.

As shown in FIG. 3, spherical aberration on the surface of Photoreceptordrums 1BK, 1M, 1Y, and 1C is generally aggravated by magnifying the beamlight without reducing it by Scanning lens 16.

Here, in Scanning optical apparatus X1, as shown in FIG. 3, Correctinglens 17 instead of Correcting lens 6 in the conventional example isemployed and arranged such that a lens axis a1 (the axis running throughthe center of a lens, while running at right angle to the surface of thesame) is displaced (hereinafter referred to as “shift”) in the subscanning direction relative to a light axis a2 of the beam light, andfurthermore, such that the lens axis a1 is inclined (hereinafterreferred to as “tilt”) relative to the light axis a2, in short,decentered Correcting lens 17 is employed, for the purpose ofsuppressing aggravated spherical aberration.

That is, such Correcting lens 17 is the same type as Correcting lens 6in the conventional example, and is merely what Correcting lens 6 isarranged as being displaced and inclined. In other words, thecross-section surface of Correcting lens 17 in the sub scanningdirection is spherical surface shape with its incident surface andoutputting surface of the beam light having a fixed curvature. However,even with such a spherical surface shape lens, suppression of sphericalaberration works well, compared with using Correcting lens 6 arrangedwithout being decentered. Correcting lens 17 is one example of adecentered lens.

FIG. 11 shows a schematic diagram for describing definition of shift andtilt. Hereinafter, as referring to FIG. 11, a shift amount of Lens axisa1 relative to Light axis a2 (hereinafter referred to as “shift amountSI”), as well as the definition of a tilt angle (hereinafter referred toas “tilt amount TI”) are explained.

As shown in FIG. 11( a), the intersecting point with Lens axis a1 on thesurface of Correcting lens 17 is prescribed as an original point O. Inthis case, as in FIG. 11( b), a distance between Original point O andLight axis a2 is Shift amount SI. Also, an angle formed between Lensaxis a1 and Light axis a2 is Tilt amount TI. Thus, Shift amount SI has adistance unit, and Tilt amount TI has an angle unit.

Generally, Shift amount SI and Tilt amount TI are independently definedrelative to a single lens. That is, Correcting lens 17 (decentered lens)similar to Correcting lens 6 in the conventional example has twoindependent parameters (degree of freedom): Shift amount SI and Tiltamount TI. By using Correcting lens 17 arranged as being properly setwith these degree of freedom, spherical aberration aggravated byScanning lens 16 can be suppressed.

The following chart 1 indicates various constant numbers which determinethe optical characteristic of Scanning optical apparatus X1.Additionally, a chart 2 indicates various constant numbers whichdetermine the decentering amount of Correcting lens 17 (Shift amount SIand Tilt amount TI). Moreover, FIGS. 4( a) and (b) respectively indicatethe reaching point of the beam light to Photoreceptor drum 1 in thecases when using Correcting lens 17 arranged so as to be decentered inaccordance with various constant numbers prescribed in Chart 2, and whenusing Correcting lens 6 of a spherical surface shape arranged withoutbeing decentered as in the conventional example.

Surface numbers 1, 2, 3 and 4 indicated in Chart 1 in below respectivelycorrespond to: a surface of the incident side of the beam light fromScanning lens 16 (hereinafter referred to as “surface 1”), and a surfaceof outputting side of the same (hereinafter referred to as “surface 2”),and a surface of the incident side of the beam light from Correctinglens 17 (hereinafter referred to as “surface 3”), and a surface ofoutputting side of the same (hereinafter referred to as “surface 4”).Also, surface separation numbers 1, 2, 3, 4 and 5 respectivelycorrespond to separations: between the reflecting surface of the beamlight of Polarizer 4 and Surface 1, between Surface 1 and Surface 2,between Surface 2 and Surface 3, between Surface 3 and Surface 4, andbetween Surface 4 and the scanning surface of Photoreceptor drums 1BK,1M, 1Y, and 1C.

In addition, dashed circles illustrated in FIGS. 4( a), (b), (c), and(d) indicate the boundary of reaching point of the beam light forobtaining sufficient density of the beam light on Photoreceptor drum 1.That is, the converging condition of the beam light within the dashedcircles indicates a condition in which generally desirable density ofthe beam light is being obtained.

CHART 1 Lens refraction index surface # curvature radius Scanning1.507595 1 −5.568 lens 16 2 −7.487 Correcting 1.507595 3 57.256 lens 174 −46.853 surface separation # surface separation 1 25 2 5 3 45 4 5 5120

CHART 2 SI(mm) TI(°) Decentering amount 2.63E−01 4.06E−01

When using Correcting lens 17 arranged so as to be decentered inaccordance with various constant numbers prescribed in Chart 2, asillustrated in FIG. 4( a), the beam light is sufficiently narrowed downon Photoreceptor drum 1, and it can be seen that spherical aberration issuppressed. Thus, it is possible to increase writing speed of anelectrostatic latent image since the beam light of high density isilluminated on Photoreceptor drum 1.

On the other hand, as shown in FIG. 4( b), when using Correcting lens 6which is spherical surface shape and not decentered and employed in theconventional example, instead of decentered Correcting lens 17, it ishard to say that the beam light is sufficiently narrowed down inPhotoreceptor drum 1, and that means spherical aberration is notsufficiently suppressed.

In addition, when Scanning optical apparatus X1 is composed according toCharts 1 and 2, the sub scanning magnification β is −1. Also, accordingto Chart 1, the parameter L1/T described later is 0.4.

Here, as shown in FIG. 3, when L1 is a distance between Polarizer 4 andthe outputting side of the beam light of Correcting lens 17 on the lightaxis of the beam light, and T is a distance between Polarizer 4 and thesurface (scanning surface) of Photoreceptor drum 1, and if the followingexpression (3) is satisfied, downsizing of the apparatus as well asmaintaining sub scanning magnification β small can be sufficientlyachieved.0.3≦L1/T≦0.5  (3)

0.5 as a higher limit of L1/T indicates a boundary for sufficientlydownsizing Scanning optical apparatus X1.

0.3 as a smaller limit of L1/T indicates a boundary for maintaining subscanning magnification β small, for example, at |β|≦2. Also, when L1/Tis smaller than 0.3, spherical aberration cannot be suppressed eventhough Correcting lens 17 arranged as being decentered is used, sincethe beam light after passing through Scanning lens 16 spreads too widein the sub scanning direction. Alternatively, the shape of Correctinglens 17 needs to be a lens shape hard to be produced.

Chart 3 in below indicates various constant numbers which determine theoptical characteristics of Scanning optical apparatus X1 of whenL1/T=0.25 is satisfied. And also, Chart 4 in below indicates variousconstant numbers which determine the decentering amount of Correctinglens 17 of when L1/T=0.25 is satisfied.

Similarly, the following Chart 5 indicates various constant numberswhich determine the optical characteristics of Scanning opticalapparatus X1 of when L1/T is at its smaller limit of 0.3. And also,Chart 6 in below indicates various constant numbers which determine thedecentering amount of Correcting lens 17 of when L1/T=0.3 is satisfied.

CHART 3 Lens refraction index surface # curvature radius Scanning1.507595 1 −3.578835912 lens 16 2 −5.407954371 Correcting 1.507595 361.19355804 lens 17 4 −31.54407987 surface separation # surfaceseparation 1 10 2 5 3 30 4 5 5 150

CHART 4 SI(mm) TI(°) Decentering amount 4.77E−02 1.58E−03

CHART 5 Lens refraction index surface # curvature radius Scanning1.507595 1 −5.110258569 lens 16 2 −7.068530755 Correcting 1.507595 361.14850221 lens 17 4 −36.66532684 surface separation # surfaceseparation 1 20 2 5 3 30 4 5 5 140

CHART 6 SI(mm) TI(°) Decentering amount 1.63E−02 9.81E−03

FIG. 4( c) is a plain view indicating reaching point of the beam lightto Photoreceptor drum 1 of when the condition: L1/T=0.25 is set, andmoreover, the decentering amount of Correcting lens 17 is prescribedaccording to each of the parameters in Chart 4. In addition, FIG. 4( d)is a plain view indicating reaching point of the beam light toPhotoreceptor drum 1 of when optical characteristics of Scanning opticalapparatus X1 is prescribed according to each of the parameters in Charts5 and 6 in the above.

As shown in FIG. 4( c), it can be seen that, when L1/T is smaller than0.3, the beam light is not narrowed down on Photoreceptor drum 1 eventhough Correcting lens 17 arranged as being decentered is used. Inaddition, as shown in FIG. 4( d), when L1/T=0.2, it is a criticalsituation right before the narrowing range of the beam light scattersout from the area of the dashed circle mentioned above, and that is thelimiting point for satisfying the image quality.

Additionally, when Scanning optical apparatus X1 is composed accordingto Charts 3 and 4, as well as according to Charts 5, and 6, the subscanning magnification β is −2.

As mentioned above, for the purposes of downsizing the apparatus,decreasing the sub scanning magnification β, and reducing sphericalaberration, it is desirable to maintain L1/T within the range of 0.3 to0.5.

Scanning optical apparatus X1 according to one embodiment of the presentinvention shows that the reduction starting point of the beam light,which was in the position of Scanning lens 5 (corresponding to Scanninglens 16) in the conventional example, shifts to the side of Correctinglens 6 (corresponding to Correcting lens 17) by predetermining therefractive power of Scanning lens 16 such that the light flux of thebeam light is magnified by Scanning lens 16. Therefore, it is possibleto suppress sub scanning magnification β defined by Scanning lens 16 andCorrecting lens 17 small.

In addition, even when sub scanning magnification β same as that of theconventional example is obtained, it is still possible to put bothScanning lens 16 and Correcting lens 17 relatively in the side ofPolarizer 4. This, as shown in FIG. 2, makes it possible for the beamlight scanned in the main scanning direction by Polarizer 4 fall onScanning lens 16 and Correcting lens 17 before its scanning range in themain scanning direction spreads too wide, thereby preventing Scanninglens 16 and Correcting lens 17 from being elongated in the main scanningdirection.

In addition, when the beam light is magnified in the sub scanningdirection, it becomes difficult to suppress spherical aberration on thescanning surface. However, as mentioned above, it is possible tosuppress the spherical aberration aggravated by such magnification smallby arranging Correcting lens 17 decentered relative to Light axis a2 ofthe beam light, and thus, density growth of the beam light onPhotoreceptor drum 1 can be achieved.

Embodiment

In the above embodiment, an example in which a beam light is magnifiedin a sub scanning direction by previously determining the refractivepower of Scanning lens 16 at negative value is described, however, it isnot intended to limit the scope of this invention. That is, as shown inFIG. 5, it is possible to shift the reduction starting point of the beamlight from Scanning lens 16 to the side of Correcting lens 17, eventhought the refractive power of Scanning lens 16 is predetermined atpositive but small value with which the beam light will not converge ina sub scanning direction. Even in such case, it is still possible todecrease the sub scanning magnification β.

In addition, the following occurs when refractive power of Scanning lens16 is predetermined at a small but positive value with which the lightflux of the beam light does not converge in the sub scanning direction.That is, as shown in FIG. 6, by refracting the beam light by Scanninglens 16, the position of Polarizer 4 as an outputting position of thebeam light relative to Scanning lens 16 shifts from an actual positionto a hypothetical position farer from Scanning lens 16 than the actualposition. This allows the optical characteristics same as that of whenthe distance between Polarizer 4 and Scanning lens 16 (and Correctinglens 17) is fixed long to be obtained, and more particularly, the subscanning magnification β can be decreased.

In the above embodiment, the example in which Scanning optical apparatusX1 according to one embodiment of the present invention is applied to aprinter is described, however, the present invention can be applicableto various image forming apparatuses, such as copiers, facsimiles, andMFPs (Multi Function Products).

And also, in the above embodiment, Scanning lens 16 and Correcting lens17 are respectively consisted of a single lens, however, it is notintended to limit the scope of this invention. That is, each of Scanninglens 16 and Correcting lens 17 may be consisted of a plurality oflenses, and the function of Scanning lens 16 and Correcting lens 17mentioned above may be realized by such plurality of lenses.

In the above embodiment, as indicated in Chart 2, both shift and tiltare employed as mentioned above, however, it is not intended to limitthe scope of this invention. That is, only one of shift and tilt whichseems more proper can be employed in order to suppress sphericalaberration.

1. A scanning optical apparatus comprising: a first lens system forconverging a beam light outputted from a prescribed light source whichoutputs a beam light; a beam light scanning means for scanning said beamlight on a prescribed scanning surface as reflecting said beam light onor near a converging point of said beam light defined by said first lenssystem; and a second lens system for converging said beam light scannedby said beam light scanning means in a sub scanning directionintersecting at least with a traveling direction of said beam light anda main scanning direction of said beam light on said scanning surface,wherein said second lens system comprises a magnifying lens formagnifying said beam light in said sub scanning direction, beingarranged in the side of said beam light scanning means, and a decenteredlens for converging said beam light on said scanning surface in said subscanning direction, being arranged in the side of said scanning surfacewith an axis of said decentered lens being decentered with respect to alight axis of said beam light.
 2. A scanning optical apparatus accordingto claim 1, wherein said magnifying lens has a negative refractive powerin said sub scanning direction.
 3. A scanning optical apparatusaccording to claim 1, wherein said decentered lens has a sphericalsurface shape formed such that curvature of an incident surface and anoutputting surface of said beam light is nearly-fixed relative to saidsub scanning direction.
 4. A scanning optical apparatus according toclaim 2, wherein said decentered lens has a spherical surface shapeformed such that curvature of an incident surface and an outputtingsurface of said beam light is nearly-fixed relative to said sub scanningdirection.
 5. A scanning optical apparatus according to claim 1, whereinan expression: 0.3≦L1/T≦0.5 is satisfied, in which L1 is a distance fromsaid beam light scanning means to said decentered lens on a light axisof said beam light, and T is a distance between said beam light scanningmeans and said scanning surface.
 6. A scanning optical apparatusaccording to claim 2, wherein an expression: 0.3≦L1/T≦0.5 is satisfied,in which L1 is a distance from said beam light scanning means to saiddecentered lens on a light axis of said beam light, and T is a distancebetween said beam light scanning means and said scanning surface.
 7. Ascanning optical apparatus according to claim 3, wherein an expression:0.3≦L1/T≦0.5 is satisfied, in which L1 is a distance from said beamlight scanning means to said decentered lens on a light axis of saidbeam light, and T is a distance between said beam light scanning meansand said scanning surface.
 8. A scanning optical apparatus according toclaim 4, wherein an expression: 0.3≦L1/T≦0.5 is satisfied, in which L1is a distance from said beam light scanning means to said decenteredlens on a light axis of said beam light, and T is a distance betweensaid beam light scanning means and said scanning surface.
 9. A scanningoptical apparatus according to claim 1, further comprising a pluralityof light sources which respectively output said beam light.
 10. Ascanning optical apparatus according to claim 2, further comprising aplurality of light sources which respectively output said beam light.11. A scanning optical apparatus according to claim 3, furthercomprising a plurality of light sources which respectively output saidbeam light.
 12. A scanning optical apparatus according to claim 4,further comprising a plurality of light sources which respectivelyoutput said beam light.
 13. A scanning optical apparatus according toclaim 5, further comprising a plurality of light sources whichrespectively output said beam light.
 14. A scanning optical apparatusaccording to claim 6, further comprising a plurality of light sourceswhich respectively output said beam light.
 15. A scanning opticalapparatus according to claim 7, further comprising a plurality of lightsources which respectively output said beam light.
 16. A scanningoptical apparatus according to claim 8, further comprising a pluralityof light sources which respectively output said beam light.
 17. Ascanning optical apparatus according to claim 1, said axis Of saiddecentered lens is inclined with respect to said light axis of said beamlight.